1. Field of the Invention
The present invention relates to a Liquid crystal Display (LCD) device and more particularly, to an active-matrix addressing LCD device of the lateral electric field type, such as the In-Plane Switching (IPS) type.
2. Description of the Related Art
Generally, the LCD device has features, such as low profile, reduced weight, and low power consumption. In particular, the active-matrix addressing LCD device that drives the respective pixels arranged in a matrix array by the active elements has ever been recognized as a high image quality flat-panel display device. Especially, the active-matrix addressing LCD device using thin-film transistors (TFTs) as the active elements for switching the respective pixels has been extensively diffused.
Most of active-matrix addressing LCD devices, which utilizes the electrooptic effects of the TN (Twisted Nematic) type liquid crystal material sandwiched by two substrates, display images by the application of an electric field approximately vertical to the main surfaces of the substrates across the liquid crystal material to thereby cause displacement of the liquid crystal molecules. These LCD devices are termed the “vertical electric field” type. On the other hand, some of the active-matrix addressing LCD devices display images by the application of an electric field approximately parallel to the main surfaces of the substrates across the liquid crystal material to thereby cause displacement of the liquid crystal molecules in the planes parallel to the main surfaces. These LCD devices have been known also, which are termed the “lateral electric field” type. Various improvements have ever been made not only for the vertical electric field type LCD devices but also for the lateral electric field type ones. Some of the improvements made for the latter will be exemplified below.
A structure using comb-tooth-like electrodes mated with each other in the lateral electric field type LCD device is disclosed in the Patent Document 1 (i.e., U.S. Pat. No. 3,807,831) issued in 1974 (see claim 1, FIGS. 1 to 4 and FIG. 11).
A technique using comb-tooth-like electrodes mated with each other similar to those in the Patent Document 1 in the active-matrix addressing LCD device utilizing the electrooptic effects of the TN type liquid crystal material is disclosed in the Patent Document 2 (i.e., Japanese Unexamined Patent Publication No. 56-091277) published in 1981 (see claim 2, FIG. 7 and FIGS. 9 to 13). This technique reduces the parasitic capacitance between the common electrode(s) and the drain bus lines, or that between the common electrode(s) and the gate bus lines.
A technique that realizes a lateral electric field type LCD device without using the comb-tooth-like electrodes in the active-matrix addressing LCD device using TFTs is disclosed in the Patent Document 3 (i.e., Japanese Unexamined Patent Publication No. 7-036058) published in 1995 (see claims 1 and 5, FIGS. 1 to 23). With this technique, the common electrode(s) and the image signal electrodes or the common electrode(s) and the liquid crystal driving electrodes are formed on different layers in such a way that an insulating film intervenes between them, and at the same time, the common electrode(s) or the liquid crystal driving electrodes is/are formed to be ring-, cross-, T-, Π-, H-, or ladder-shaped.
A technique that the comb-tooth-like electrodes for driving the liquid crystal material (i.e., the pixel electrodes and the common electrode(s)) are formed by a transparent conductive material or materials in such a way as to be placed on an upper layer than the drain bus lines (or the data bus lines), in other words, to be placed nearer to the liquid crystal layer than the drain or data bus lines, is disclosed in the Patent Document 4 (i.e., Japanese Unexamined Patent Publication No. 2001-222030) published in 2001 (see abstract and FIGS. 3 to 5).
FIG. 1 is a partial plan view showing an example of the structure of the first substrate (i.e., the active-matrix substrate) used in the related-art lateral electric field type LCD device disclosed in the Patent Document 4. FIG. 2 is a cross-sectional view of the LCD device along the line II-II in FIG. 1. These two figures show the structure of one of the pixel regions.
The first substrate 111 of the related-art LCD device shown in FIGS. 1 and 2 comprises gate bus lines 155 that are extended along the lateral (horizontal) direction of FIG. 1 and arranged at equal intervals along the longitudinal (vertical) direction of the same figure, and drain bus lines 156 that are extended along the longitudinal direction of FIG. 1 and arranged at equal intervals along the lateral direction of the same figure. A pixel region P is formed in each of the approximately rectangular areas defined by the gate bus lines 155 and the drain bus lines 156. These pixel regions P (i.e., the pixels) are arranged in a matrix array as a whole.
Further, the first substrate 111 of this related-art LCD device comprises common bus lines 152, each of which is extended in parallel to a corresponding one of the gate bus lines 155. These common bus lines 152 are provided for electrical interconnection among the common electrodes 172 formed in the respective pixel regions P. Each of the common bus lines 152 is located near the upper end of each pixel region P and is apart from a corresponding one of the gate bus lines 155 at a predetermined distance. The gate bus lines 155, the drain bus lines 156, and the common bus lines 152 are made of opaque metallic materials, respectively.
In each of the pixel regions P, the corresponding common bus line 152 comprises two belt-shaped light-shielding parts 152a that extend respectively along the two drain bus lines 156 that define the pixel region P. These two light-shielding parts 152a are united with the common bus line 152. The light-shielding part 152a positioned at the left side of the pixel region P is adjacent to the right edge of the drain bus line 156 placed at the left side of the pixel region P. The light-shielding part 152a positioned at the right side of the pixel region P is adjacent to the left edge of the drain bus line 156 placed at the right side of the pixel region P. These two light-shielding parts 152a have the same plan shape or pattern.
For each of the pixel regions P, a TFT 145 is formed near the intersection of the corresponding gate bus line 155 and the corresponding drain bus line 156. The TFT 145 is formed by a gate electrode (not shown) united with the corresponding gate bus line 155; an island-shaped semiconductor film 143 overlapped with the gate electrode in such a way that a gate insulating film 157 intervenes between them; a drain electrode 156a united with the corresponding drain bus line 156 and overlapped with the semiconductor film 143; and a source electrode 142 formed to be opposite to the drain electrode 156a at a predetermined distance and overlapped with the semiconductor film 143. The gate electrode, the drain electrode 156a, and the source electrode 142 are made of opaque metallic materials, respectively.
In each of the pixel regions P, a pixel electrode 171 and a common electrode 172 for generating liquid crystal driving electric field are formed. The pixel electrode 171 and the common electrode 172, each of which is made of a transparent conductive material, have comb-tooth-like plan shapes.
The pixel electrode 171 comprises a belt-shaped base 171b placed on the side of the TFT 145 (i.e., the TFT side) in the pixel region P, and four comb-tooth-like parts 171a protruding from the base 171b toward the opposite side to the TFT 145 (toward the upper side in FIG. 1) in the pixel region P. The four comb-tooth-like parts 171a are extended in parallel to the drain bus lines 156 and are arranged along the base 171b (along the lateral direction in FIG. 1) at equal intervals. The top ends of the respective comb-tooth-like parts 171a are located near the corresponding common bus line 152. Two of the comb-tooth-like parts 171a placed at outer positions are respectively overlapped with the corresponding light-shielding parts 152a existing near the two drain bus lines 156 that define the pixel region P. The pixel electrode 171 is electrically connected to the source electrode 142 of the corresponding TFT 145 at the base 171b by way of a corresponding one of contact holes 161.
The common electrode 172 comprises a belt-shaped base 172b placed on the opposite side of the TFT 145 in the pixel region P, and three comb-tooth-like parts 172a protruding from the base 172b toward the side of the TFT 145 (toward the lower side in FIG. 1) in the pixel region P. The three comb-tooth-like parts 172a are extended in parallel to the drain bus lines 156 and are arranged along the base 172b (along the lateral direction in FIG. 1) at equal intervals. The top ends of the respective comb-tooth-like parts 172a are located near the base 171b of the pixel electrode 171. The three comb-tooth-like parts 172a and the four comb-tooth-like parts 171a are arranged alternately along the gate and common bus lines 155 and 152. Therefore, it may be said that these comb-tooth-like parts 172a and 171a are mated with each other. The common electrode 172 is electrically connected to the corresponding common bus line 152 at the base 172b by way of a corresponding one of contact holes 162.
As shown in FIG. 2, this related-art LCD device comprises the first substrate (i.e., the active-matrix substrate) 111 having the structure of FIG. 1, a second substrate (i.e., an opposite substrate) 112 opposed to the first substrate 111 at a predetermined gap, and a liquid crystal layer 120 placed between the substrates 111 and 112.
The gate bus lines 155, the common bus lines 152, the light-shielding parts 152a, and the gate electrodes of the TFTs 145 are formed on the surface of the glass plate 111a of the first substrate 111. The gate insulating film 157 is formed on the surface of the glass plate 111a to cover the gate bus lines 155, the common bus lines 152, the light-shielding parts 152a, and the gate electrodes. (Only the light-shielding parts 152a are shown in FIG. 2.) Each of the gate electrodes is united with a corresponding one of the gate bus lines 155. The drain bus lines 156, the semiconductor films 143 of the TFTs 145, the drain electrodes 156a, and the source electrodes 142 are formed on the gate insulating film 157. A protective insulating film 159 is formed on the gate insulating film 157 to cover the drain bus lines 156, the semiconductor films 143, the drain electrodes 156a, and the source electrodes 142. (Only the drain bus lines 156 are shown in FIG. 2.) The pixel electrodes 171 and the common electrodes 172 are formed on the protective insulating film 159. (Only the comb-tooth-like parts 171a of the pixel electrode 171 and the comb-tooth-like parts 172a of the common electrode 172 are shown in FIG. 2.)
As explained above, the common bus lines 152 are located on the surface of the glass plate 111a and the drain bus lines 156 are located on the gate insulating film 157. Therefore, the common bus lines 152 are placed on a lower layer than the drain bus lines 156, in other words, on a further layer from the liquid crystal layer 120 than the drain bus lines 156. Similarly, since the gate bus lines 155 are located on the surface of the glass plate 111a, the gate bus lines 155 also are placed on a lower layer than the drain bus lines 156, in other words, on a further layer from the liquid crystal layer 120 than the drain bus lines 156. Because the pixel electrodes 171 and the common electrodes 172 are formed on the protective insulating film 159, the pixel and common electrodes 171 and 172 are placed on an upper layer than the drain bus lines 156, in other words, on a nearer layer to the liquid crystal layer 120 than the drain bus lines 156.
On the surface (i.e., the inner face) of the first substrate 111 having the above-described structure, in other words, on the protective insulating film 159, an alignment film 131 made of an organic polymer is formed. Therefore, the pixel electrodes 171 and the common electrodes 172 are covered with the alignment film 131. The surface of the alignment film 131 is subjected to a predetermined aligning treatment.
On the other hand, a color layer 182 including the three primary colors of red (R), green (G) and blue (B) is formed on the surface of the glass plate 112a of the second substrate 112 corresponding to the respective pixel regions P. A light-shielding black matrix layer 181 is formed on the surface of the glass plate 112a in the remaining region excluding the regions corresponding to the pixel regions P. The color layer 182 is formed by a red color sublayer 182R, a green color sublayer 182G, and a blue color sublayer 182B, each of which has been patterned to have a predetermined shape. The plan shape or pattern of the black matrix 181 is determined in such a way as to cover the structural elements formed on the first substrate 111 by opaque metallic materials (i.e., the gate bus lines 155, the drain bus lines 156, the common bus lines 152, the light-shielding parts 152a, and the TFTs 145) and to define rectangular openings (i.e., light transmission areas) in the respective pixel regions P. The color layer 182 is selectively placed in these openings or light transmission areas.
The color layer 182 and the black matrix layer 181 are covered with an overcoat film 185. The overcoat film 185, which is formed to cover the whole surface of the glass plate 112a, is provided to protect the color layer 182 and the black matrix layer 181 and to planarize the level differences generated by the color layer 182 and the black matrix layer 181. Columnar spacers (not shown) are formed on the black matrix layer 181 to keep the gap between the first and second substrates 111 and 112.
On the surface (i.e., the inner face) of the second substrate 112 having the above-described structure, in other words, on the overcoat film 185, an alignment film 132 made of an organic polymer is formed. Therefore, the columnar spacers are covered with the alignment film 132. The surface of the alignment film 132 is subjected to a predetermined aligning treatment.
The first substrate (i.e., the active-matrix substrate) 111 and the second substrate (i.e., the opposite substrate) 112 are superposed on each other at a predetermined gap in such a way that their surfaces on which the alignment films 131 and 132 are respectively formed are directed inward and opposed to each other. A liquid crystal material is confined in the space between the substrates 111 and 112, forming the liquid crystal layer 120. In other words, the liquid crystal layer 120 is sandwiched and held by the substrates 111 and 112. A pair of polarizer plates (not shown) is arranged on the outer surfaces of the first and second substrates 111 and 112 (i.e., the backs of the glass plates 111a and 112a), respectively.
Because the surfaces of the alignment films 131 and 132 have been subjected to the predetermined aligning treatments, the liquid crystal molecules 121 existing in the liquid crystal layer 120 are aligned in parallel along a predetermined direction shifted at a fixed angle (e.g., approximately 15° clockwise) from the vertical or longitudinal direction of FIG. 1 when no electric field is applied, as shown by the arrow in FIG. 1. This means that the initial alignment direction of the liquid crystal molecules 121 is defined at the direction indicated by the arrow in FIG. 1. Moreover, the transmission axes of the pair of polarizer plates are crossed at right angles. The transmission axis of one of the pair of polarizer plates is in accordance with the alignment direction of the liquid crystal molecules 121 when no electric field is applied (i.e., the initial alignment direction).
Next, a method of fabricating the related-art LCD device shown in FIGS. 1 and 2 will be explained below.
The first substrate 111 is fabricated in the following way.
First, a chromium (Cr) film is formed on the whole surface of the glass plate 111a and patterned to have a predetermined shape, thereby forming the gate bus lines 155, the common bus lines 152, and the light-shielding parts 152a on the surface of the glass plate 111a. At this time, the gate electrodes also are formed in such a way as to be united with the corresponding gate bus lines 155. Next, the gate insulating film 157, which is made of silicon nitride (SiNx), is formed on the whole surface of the glass plate 111a to cover the gate electrodes, the gate bus lines 155, the common bus lines 152, and the light-shielding parts 152a. 
Subsequently, an amorphous silicon (a-Si) film is formed on the gate insulating film 157 and patterned to result in island-like parts, thereby forming the island-shaped semiconductor films 143 for the TFTs 145. Each of the island-shaped semiconductor films 143 is overlapped with a corresponding one of the gate bus lines 155 in such a way that the gate insulating film 157 intervenes between them. Moreover, a Cr film is formed on the gate insulating film 157 and patterned, thereby forming the drain bus lines 156, the drain electrodes 156a, and the source electrodes 142 on the gate insulating film 157. Thereafter, the protective insulating film 159, which is made of SiNx, is formed on the whole surface of the glass plate 111a, covering the drain bus lines 156, the drain electrodes 156a, and the source electrodes 142.
Following this, the protective insulating film 159 is selectively removed at the predetermined positions superposed on the respective source electrodes 142, thereby forming the contact holes 161 that reach the corresponding source electrodes 142. Moreover, the protective insulating film 159 and the gate insulating film 157 are selectively removed at the predetermined positions superposed on the respective common bus lines 152, thereby forming the contact holes 162 that reach the corresponding common bus lines 152.
Thereafter, a transparent conductive film, which is made of ITO (Indium Tin Oxide) or the like, is formed on the protective insulating film 159 and patterned to have a predetermined shape, thereby forming the pixel electrodes 171 each having the comb-tooth-like parts 171a and the common electrodes 172 each having the comb-tooth-like parts 172a on the protective insulating film 159. At this time, the pixel electrodes 171 are electrically connected to the corresponding source electrodes 142 by way of the corresponding contact holes 161. The common electrodes 172 are electrically connected to the corresponding common bus lines 152 by way of the corresponding contact holes 162. In this way, the first substrate 111 is fabricated.
The second substrate 112 is fabricated in the following way.
First, the black matrix layer 181 and the color layer 182 each having the predetermined shape or pattern are formed on the surface of the glass plate 112a. When the color layer 182 is formed, the red color sublayer 182R, the green color sublayer 182G, and the blue color sublayer 182B each having a predetermined shape may be successively formed on the surface of the glass plate 112a in an appropriate order. Next, the overcoat film 185 is formed on the whole surface of the glass plate 112a, thereby covering the black matrix layer 181 and the color layer 182. Thereafter, the columnar spacers (not shown) are formed on the overcoat film 185. In this way, the second substrate 112 is fabricated.
Following this, the alignment films 131 and 132, which are made of polyimide, are respectively formed on the surface of the first substrate 111 and the surface of the second substrate 112 fabricated in the above-described manners. The surfaces of the alignment films 131 and 132 are uniformly subjected to the predetermined aligning treatment.
Thereafter, the first and second substrates 111 and 112 are superposed on each other to have a predetermined gap such as 4.0 μm. Next, in a vacuum chamber (not shown), a predetermined nematic liquid crystal material whose refractive index anisotropy is, for example, 0.075 is injected into the space between the substrates 111 and 112 and then, the space is sealed. After the sealing operation of the space is completed, the polarizer plates (not shown) are respectively adhered onto the outer surfaces of the substrates 111 and 112. As a result, the LCD panel is completed.
A predetermined driver LSI (Large-Scale Integrated Circuit) and a predetermined backlight unit are mounted on the LCD panel thus fabricated. As a result, the related-art LCD device having the structure shown in FIGS. 1 and 2 is completed.
With the related-art lateral electric field type LCD device shown in FIGS. 1 and 2, the liquid crystal driving electric field is uniformly generated along the approximately lateral (horizontal) direction of FIG. 1 when a voltage is applied. For this reason, the liquid crystal molecules 121 aligned along the initial alignment direction (i.e., the direction indicated by the arrow in FIG. 1) when no electric field is applied are rotated clockwise by the liquid crystal driving electric field, and as a result, the alignment state of the liquid crystal molecules 121 is changed. Since the transmittance in the respective pixel regions P (i.e., the respective pixels) is modulated by such the alignment state change of the liquid crystal molecules 121, images can be displayed as desired.
Moreover, with the related-art lateral electric field type LCD device shown in FIGS. 1 and 2, both of the pixel electrodes 171 and the common electrodes 172 for generating the liquid crystal driving electric field are made of a transparent conductive material and therefore, light penetrates through the regions where the pixel and common electrodes 171 and 172 are present. For this reason, the aperture ratio and the transmittance are improved compared with the case where the pixel electrodes 171 and the common electrodes 172 are made of an opaque metal or metals.
In addition, since the amount of the opaque metallic parts existing in the openings of the black matrix layer 181 is reduced in the above-described manner, the scattering of light caused by the edges of the opaque metallic parts and the alignment distortion of the liquid crystal molecules 121 caused by the level differences in the vicinities of the same edges are suppressed. The scattering of light and the alignment distortion of the liquid crystal molecules 121 will be a cause of optical leakage when black is displayed. However, with this related-art lateral electric field type LCD device, the scattering of light and the alignment distortion of the liquid crystal molecules 121 are suppressed and therefore, the display contrast is improved.
If the opaque metallic parts are present in the openings of the black matrix layer 181, the incident light will be reflected by these opaque metallic parts when this related-art LCD device is used in a comparatively well-lighted place, and as a result, the “light place contrast” is likely to deteriorate conspicuously. However, this related-art LCD device does not include such the opaque metallic parts in the openings of the black matrix layer 181. Accordingly, with this related-art LCD device, the light place contrast is prevented from deteriorating.
Furthermore, the two light-shielding parts 152a, which are united with the corresponding common bus line 152 in the pixel region P as shown in FIG. 1, are respectively provided in the vicinities of the two drain bus lines 156 that define the pixel region P. The areas between the light-shielding parts 152a and the corresponding drain bus lines 156 adjacent thereto are covered with the black matrix layer 181. Therefore, the leakage of light induced by the leaked electric field from the respective drain bus lines 156 and the vertical cross talk are restrained.
With the related-art LCD device shown in FIGS. 1 and 2, the above-described advantages are obtained. However, the two light-shielding parts 152a united with the corresponding common bus line 152 are respectively extended along the two drain bus lines 156 that define the pixel region P over a long distance in the vicinities of these drain bus lines 156. Therefore, the coupling capacitances (i.e., the parasitic capacitances) between these two drain bus lines 156 and the corresponding common bus line 152 (i.e., the corresponding common electrode 172) will be large.
For this reason, the effect of electric potential fluctuation (i.e., the effect of signal voltage change) on the drain bus lines 156 is likely to be transferred to the corresponding common electrode 172 by way of the corresponding common bus line 152. In other words, the electric potential (which should be kept constant originally) of the common electrode 172 electrically connected to the corresponding common bus line 152 tends to fluctuate due to the electric potential fluctuation on the corresponding drain bus lines 156. As a result, a problem that lateral cross talk is likely to occur arises.